Our previous studies of post-thymic T cell developmental pathways have suggested that mouse spleen cells include at lease two separate lineages of Thy 1+ progenitors--one of which is Lyt 2+ and gives rise to precursors of cytotoxic lymphocytes (pCTL) and the other of which is Lyt 2- and produces Interleukin 2 secreting helper T cells. We propose here to use limiting dilution analysis to determine the frequencies of both progenitor cell types in peripheral lymphoid tissues, and to measure the rate and extent of numerical expansion which occurs during the generation of mature effector cells. We will also ask whether the effector cells generated during post-thymic expansions differ in kind from their normal counterparts. We will also search for surface marker antigens which can distinguish progenitors from their mture progeny, or discriminate among different lineages of progenitors. Lastly, we will see if post-thymic expansion of pCTL or of helper cells can be influenced, pharmacologically, by exposure to antigen, to synthetic analogues of thymic hormones, or to Interleukins 1 and 2.